An electrostatic chuck can be employed in a plasma reactor chamber to hold a semiconductor wafer onto a wafer support surface within the chamber during plasma processing of the wafer. The electrostatic chuck consists of a flat electrode or conductive grid insulated by a flat insulating or semi-insulating layer over which the wafer is placed. Typically, thin lift pins extend upwardly through the electrostatic chuck to receive the wafer above the electrostatic chuck from a robotic device which is then retracted. The lift pins are then retracted downwardly until the wafer rests on the wafer support surface. A large D.C. chucking voltage is applied to the electrode, typically with respect to a chamber wall ground. The wafer is typically referenced to ground indirectly through the plasma. The application of the large DC voltage to the electrostatic chuck electrode with the plasma “on” produces a large electrostatic force that holds the wafer in place. Plasma processing of the wafer is then performed, after which the lift pins extend upwardly to lift the wafer from the electrostatic chuck to the robot device for removal from the chamber. For high throughput, the lift pins are moved at a relatively high speed during their downward retraction and upward extension motions. The removal of the applied D.C. chucking voltage does not necessarily eliminate the potential difference between the wafer being chucked and the surface of the electrostatic chuck. Residual charge can remain, resulting in an residual attractive force between wafer and chuck. Depending on the magnitude of the residual force and the velocity of the lift pins during their upward extension, the wafer may be broken.
Lifting the pins when the plasma is “on” provides a discharge path for the current caused by the separation of the wafer from the chuck, but if the residual force is large, the wafer may still break.
Some prior art methods use the gas flow rate of a heat transfer gas (such as helium) delivered to the surface of the electrostatic chuck at constant pressure as a measure of residual chucking force. This method generally fails when some portion of the wafer “dechucks”, allowing the heat transfer gas to leak out at high flow rate, while another portion of the wafer remains chucked and may be broken in a subsequent upward motion of the lift pins.
Currently, there is no way to detect occurrence of such an error prior to destruction of the wafer.
A related problem is that process failure (due to excessive heating or poor temperature control) may occur if the wafer is not securely chucked to the wafer support surface prior to the start of plasma processing in the chamber. Currently, there is no way of confirming the adequacy of the chucking force prior to the start of plasma processing of the wafer without supplying a heat transfer gas (such as helium) to the surface of the electrostatic chuck at constant pressure and monitoring the gas flow rate as a measure of residual chucking force. However for some applications, in particular at high-bias-voltage, heat transfer gases may break down electrically, causing the wafer to dechuck and potentially destroying the wafer and electrostatic chuck. A method of confirming the adequacy of the chucking force prior to processing is required.